The present invention relates generally to polymer resin blended with non-polymer particles, and more specifically to optical polymer blends with a low coefficient of thermal expansion.
Some systems that manipulate light require stringent alignment of optical components. Glasses are used in the majority of these applications in part because of the availability of low or negative coefficient of thermal expansion (“CTE”) glasses. Manufacturers of glasses for optical applications have developed glasses that have a negative CTE to compensate for the effects of temperature on the performance of optical systems. Polymers have been used to replace glass optical components in less demanding applications. The CTE of polymers are typically ten times higher than that of glasses. The relatively large CTE of polymers has prevented their use as optical components in the more demanding applications.
The use of inorganic fillers to decrease the CTE and moisture absorption of polymers is well known. Several patents teach the use of glass particles as fillers to improve strength and decrease the CTE of adhesive and resin systems. One technique uses particulates of fumed silica as a filler for polysiloxane resins. Silica particles of a size less than 1 micron have been incorporated in polysiloxane resin to minimize a change in refractive index. The refractive index of the resin was kept between 1.410 and 1.460, based on the d-line of sodium at 587.6 nm, and the index of the resin is close to that of the <1 micron silica particles. As a result, the particles scatter very little light and the mixture is transparent. The change in refractive index with temperature of the resin-silica mixture is improved because the relatively low CTE of the silica lowers the change in bulk density of the resin blend with temperature. Adhesives used for bonding telecommunications fibers in connectors have similarly incorporated submicron mineral particles to modify the CTE and match the refractive index of adhesive to the optical system.
Other mixtures add glass beads to thermoplastic resins to decrease the CTE and moisture absorption characteristics of the resins. The refractive index of the glass beads are adjusted to match the refractive index of the thermoplastic resin matrix. However, a difference in refractive index between the thermoplastic resin matrix and glass beads of +/−0.01065 results in a diffusing material with light scattering properties because of the slight mismatch. The glass beads had diameters in the order of 10 mm.
Organic acrylic monomers have been blended with inorganic salts of acrylic acid monomers to adjust the refractive index of coating material used to form optical waveguides. The inorganic portion of the molecule segregate into particles like domains. These domains have localized refractive index different from the bulk.